Lightning protection system

Abstract

A wind turbine blade comprising: a first blade portion having a shell that defines a suction side, a pressure side, a leading edge, a trailing edge, and a first spar cap portion, the first blade portion further including a first blade portion end surface at one end of the first blade portion; a second blade portion having a shell that defines a suction side, a pressure side, a leading edge, a trailing edge, and a second spar cap portion, the second blade portion further including a second blade portion end surface at one end of the second blade portion, wherein the first blade portion and the second blade portion are configured to be coupled together at the first and second blade portion end surfaces; a connection joint for coupling the first and second blade portions together, wherein the connection joint includes a connector for connecting to the first blade portion end surface and to the second blade portion end surface, the connector including electrically conductive material; and a lightning protection system including a down conductor portion in each of the first and second blade portions, wherein there is a gap between the connector and at least one of the down conductor portions, the lightning protection system further comprising an electrical cable electrically bonding the connector to the at least one down conductor portion across the gap.

Description

[0001] LIGHTNING PROTECTION SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a lightning protection system for a wind turbine.

[0004] BACKGROUND OF THE INVENTION

[0005] Wind turbines are susceptible to lightning strikes, and the blades of wind turbines are particularly susceptible to lightning strikes.

[0006] As a result, it is common for a wind turbine blade to include a lightning protection system that electrically couples the wind turbine blade to ground. This lightning protection system may include lightning receptors and conductors that are electrically connected from the blade, through the tower and nacelle, to ground.

[0007] Where gaps exist between electrically conductive components in wind turbine blades a difference in potential between the components may occur in the event of a lightning strike. There is therefore a risk of electric current arcing between the components across the gap, releasing energy which could damage the blade. The present invention seeks to address this issue.

[0008] SUMMARY OF THE INVENTION

[0009] A first aspect of the invention provides a wind turbine blade comprising: a first blade portion having a shell that defines a suction side, a pressure side, a leading edge, a trailing edge, and a first spar cap portion, the first blade portion further including a first blade portion end surface at one end of the first blade portion; a second blade portion having a shell that defines a suction side, a pressure side, a leading edge, a trailing edge, and a second spar cap portion, the second blade portion further including a second blade portion end surface at one end of the second blade portion, wherein the first blade portion and the second blade portion are configured to be coupled together at the first and second blade portion end surfaces; a connection joint for coupling the first and second blade portions together, wherein the connection joint includes a connector for connecting to the first blade portion end surface and to the second blade portion end surface, the connector including electrically conductive material; and a lightning protection system including a down conductor portion in each of the first and second blade portions, wherein there is a gap between the connector and at least one of the down conductor portions, the lightning protection system further comprising an electrical cable electrically bonding the connector to the at least one down conductor portion across the gap.

[0010] In the event of a lightning strike upon a wind turbine blade electric current may flow through any electrically conductive components within the blade. Where there are gaps between adjacent electrically conductive components the electric current may arc between the components, releasing energy as heat in the process. This release of energy may damage components. By electrically bonding the connector to the down conductor portion there is provided an electrical pathway through which electric current can flow, reducing the likelihood of current arcing between the connector and the down conductor portion and causing damage.

[0011] Preferably, the electrical connection comprises an electrical cable.

[0012] The gap may include a spanwise gap. The gap may be less than 2 metres, or less than 1 m, or less than 0.5 m, for example.

[0013] The down conductor portion of the first and / or second blade portions may include a metal foil layer. The metal foil layer may comprise an expanded metal foil or a metal mesh.

[0014] A metal foil layer may provide a convenient down conductor portion for covering and hence protecting a large surface area of the wind turbine blade.

[0015] The lightning protection system may include at least four of the down conductor portions each provided as metal foil layers respectively extending over: the suction side of the first spar cap portion on the suction side of the first blade portion; the pressure side of the first spar cap portion on the pressure side of the first blade portion; the suction side of the second spar cap portion on the suction side of the second blade portion; and the pressure side of the second spar cap portion on the pressure side of the second blade portion.

[0016] Providing a metal foil layer extending over each of the spar cap portions may provide good coverage for protecting all of the spar cap portions from a lightning strike. The lightning protection system may further include a plurality of electrical cables electrically bonding each of the down conductor portions to the connector.

[0017] The electrical cables may provide a convenient method for electrically bonding components spaced apart with minimal current resistance losses and hence minimal potential difference between the components, thus minimising the likelihood of electrical current arcing between them.

[0018] The connector may be adapted to transfer load between the first spar cap portion of the first blade portion and the second spar cap portion of the second blade portion.

[0019] The first spar cap portion and the second spar cap portion may be the main longitudinal load bearing structures of the wind turbine blade. As such they may provide the most appropriate interface to couple the connector to for transferring loads therebetween.

[0020] The connector may include a first connector portion located toward the leading edges of the first and second blade portions and a second connector portion located towards the trailing edges of the first and second blade portions, with a chordwise gap between the first and second connector portions.

[0021] The second connector portion may be adapted to transfer load between the trailing edge regions of the first and second blade portions. The trailing regions may have a longitudinal load bearing structure such as a rear spar or stringer. The second connector portion may be adapted to transfer load between the trailing edge region of the first blade portion and the trailing edge of the second blade portion. Providing the connector as a first connector portion spaced away from a second connector portion may allow the overall connector to be made smaller and lighter. Further the chordwise gap between the two connector portions may provide a convenient space to help with the packaging and assembly of the joint between the first and second blade portions.

[0022] The lightning protection system may further comprise an electrical cable electrically bonding the first connector portion to the second connector portion across the chordwise gap. The electrical cable may provide a convenient way of electrically bonding the first and second connector portions and minimising the likelihood of arcing occurring across the chordwise gap between the first and second connector portions.

[0023] The metal foil layer may have a projection extending towards the chordwise gap between the first and second connector portions, and the electrical cable may be coupled to the metal foil layer at the projection.

[0024] A projection may be used to reduce the distance between the metal foil layers and the connector. This may reduce the overall package size of the connection between the first and second blade portions and also allow a shorter electrical cable to be used to electrically bond the metal foil layer to the connector.

[0025] The projection may be formed as a patch of metal foil partially overlaying and in intimate electrical contact with a main portion of the metal foil layer.

[0026] There may be a need to accurately position an electrical connection point for the electrical cable to connector to the metal foil layer. Providing such an electrical connection point on a patch may allow the position of the patch and hence the electrical connection point to be more accurately controlled, with tolerance being taken up by the amount of overlap between the patch and the metal foil layer. Providing the patch may also improve handling and reduce the cost of the metal foil layer as the metal foil layer can have a more regular shape.

[0027] The first blade portion may comprise a first set of inserts embedded in the first spar cap portion, and the second blade portion may comprise a second set of inserts embedded in the second spar cap portion. The connector may be for fastening to the first set of inserts and to the second set of inserts, wherein the inserts each include electrically conductive material.

[0028] Inserts embedded in the spar cap portions provide a convenient interface into which fasteners can be coupled to fasten the connector to the first and second blade portions. Providing inserts including, e.g. metallic material, means each insert is electrically conductive. Since the connector is electrically conductive, if the fasteners are also electrically conductive (e.g. metallic) and are directly coupled between the inserts and the connector then the inserts become electrically bonded to the connector. The risk of an electrical arc forming across the gap between the inserts and the metal foil layer is therefore minimised.

[0029] The wind turbine blade may have a thickness direction and the inserts may not be directly beneath the metal foil layer in the thickness direction from the outside towards the inside of the blade.

[0030] The first and / or second spar cap portions may include carbon fibre reinforced material, which is terminated at a distance away from the connection joint. The lightning protection system may include a down conductor portion provided as a metal foil layer extending over the carbon fibre reinforced material. An equipotential bonding element may electrically bond the metal foil layer to the carbon fibre reinforced material.

[0031] Providing a metal foil layer extending over the carbon fibre reinforced material may minimise the risk of electrical current arcing to or from the ends of the carbon fibre reinforced material. Equipotential bonding elements may provide a convenient method of electrically bonding the carbon fibre reinforced material to the metal foil layer / down conductor portion to minimise the risk of a difference in potential between the carbon fibre reinforced material and the metal foil.

[0032] The carbon fibre reinforced material may include pultruded carbon.

[0033] The carbon fibre reinforced material may be terminated further away from the connection joint than the metal foil layer.

[0034] By terminating the carbon fibre reinforced material further away from the connection joint than the metal foil layer, lightning current in the metal foil layer is promoted to remain in the metal foil layer (and not in the carbon fibre reinforced material) near the joint.

[0035] Where a metal extends over a spar cap or over carbon fibre reinforced material this means that the metal foil is located between that element and the external surface of the blade. Preferably, the metal foil is located at or just under the external surface of the blade. The first blade portion may be an inboard blade part comprising the root end, and the second blade portion may be an outboard blade part comprising the tip end.

[0036] Optionally, the first spar cap portion of the inboard blade part may not comprise any carbon fibre reinforced material and the down conductor portion of the inboard blade part may not comprise a metal foil layer.

[0037] The blade construction of the inboard and outboard parts of the blade may differ. The inboard blade part may not require carbon fibre reinforced material, in which case there may be no requirement for a metal foil layer down conductor (instead a down conductor cable inside the blade may be used). In such circumstances only the outboard part of the blade may have the metal foil layer connected to the connector joining the inboard and outboard blade parts, and the down conductor of the inboard blade part may be electrically connected to the connector differently.

[0038] BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0040] Figure 1 shows a view of a wind turbine;

[0041] Figure 2 shows a wind turbine blade;

[0042] Figure 3 shows an exploded view of a connection joint according to a first example;

[0043] Figure 4 shows an exploded view of a connection joint according to a second example; Figure 5A shows a plan view of the second example of the connection joint;

[0044] Figure 5B shows a cross sectional view of the plan view of Figure 5A; and Figure 6 shows equipotentially bonded components.

[0045] DETAILED DESCRIPTION OF EMBODIMENT(S)

[0046] In this specification, terms such as leading edge, trailing edge, pressure surface, suction surface, thickness, and chord are used. While these terms are well known and understood to a person skilled in the art, definitions are given below for the avoidance of doubt.

[0047] The term leading edge is used to refer to an edge of the blade which will be at the front of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor. The term trailing edge is used to refer to an edge of a wind turbine blade which will be at the back of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

[0048] The chord of a blade is the straight line distance from the leading edge to the trailing edge in a given cross section perpendicular to the blade spanwise direction. The term chordwise is used to refer to a direction from the leading edge to the trailing edge, or vice versa.

[0049] A pressure surface (or windward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which, when the blade is in use, has a higher pressure than a suction surface of the blade.

[0050] A suction surface (or leeward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which will have a lower pressure acting upon it than that of a pressure surface, when the blade is in use.

[0051] The thickness of a wind turbine blade is measured perpendicularly to the chord of the blade and is the greatest distance between the pressure surface and the suction surface in a given cross section perpendicular to the blade spanwise direction.

[0052] The term spanwise is used to refer to a direction from a root end of a wind turbine blade to a tip end of the blade, or vice versa. When a wind turbine blade is mounted on a wind turbine hub, the spanwise and radial directions will be substantially the same.

[0053] The term spar cap is used to refer to a longitudinal, generally spanwise extending, reinforcing member of the blade. The spar cap may be embedded in the blade shell or may be attached to the blade shell. The spar caps of the windward and leeward sides of the blade may be joined by one or more shear webs extending through the interior hollow space of the blade. The blade may have more than one spar cap on each of the windward and leeward sides of the blade. The spar cap may form part of a longitudinal reinforcing spar or support member of the blade. In particular, the spar caps may form part of the load bearing structure extending in the longitudinal direction that carries the flap-wise bending loads of the blade. The spar cap may comprise spar cap portions either side of a connection joint between portions of the blade in the case of a segmented or split wind turbine blade. The term outboard refers to a radial (blade spanwise) direction from hub of the blade towards the tip end of the blade. The term inboard refers to a radial direction from the tip end of the blade towards the hub.

[0054] Figures 1 shows a view of a wind turbine 1 according to a first example. The wind turbine 1 comprises a tower 2 and a nacelle 3 mounted on the tower 2. A hub 4 is mounted rotatably on the nacelle 3, and carries three wind turbine blades 5 projecting outwardly from the nacelle 3. While the example shown in Figure 1 has three blades 5, it will be appreciated that other numbers of blades 5 are possible.

[0055] When wind blows against the wind turbine 1 , the wind turbine blades 5 generate a lift force which causes a generator (not shown) within the nacelle 3 to generate electrical energy.

[0056] It will be appreciated that the wind turbine 1 depicted may be any suitable type of wind turbine 1. The wind turbine 1 shown is an upwind wind turbine, although it will be appreciated the wind turbine 1 may be a downwind wind turbine. The wind turbine 1 may be an onshore wind turbine such that the foundation is embedded in the ground, or the wind turbine 1 may be an offshore installation in which case the foundation would be provided by a suitable marine platform.

[0057] As shown in Figure 2, the wind turbine blades 5 have a root end 11 proximal to the hub 4, adapted to be connected to the hub 4, and a tip end 12 distal from the hub 4. The blades 5 include a leading edge 13 and a trailing edge 14 that extend between the respective root end 11 and tip end 12. The blades 5 include a blade shell that defines a suction side 15 and a pressure side 16 around the blade. A thickness dimension of the blade 5 extends between the suction side 15 and the pressure side 16.

[0058] Each blade 5 may have a cross section which has a substantially circular profile near the root end 11. The blade 5 may transition from a circular profile to an aerofoil profile moving from the root end 11 of the blade 5 outboard. The blade 5 may comprise a "shoulder" 28 outboard of the root end 11 , which is the widest part of the blade where the blade 5 has its maximum chord. The blade 5 may have an aerofoil profile of progressively decreasing thickness in an outboard portion of the blade. The progressively decreasing thickness may extend from the shoulder 28 to the tip end 12. Each of the blades 5 is a split blade formed of a first blade portion 22 and a second blade portion 24 coupled together, such as shown in Figure 2. Each blade portion 22, 24 has a shell that defines a respective leading edge 30a, 30b, trailing edge 32a, 32b, suction side 34a, 34b, and pressure side 36a, 36b.

[0059] The first portion 22 and second portion 24 of each blade 5 may be connected at a connection joint indicated by connection line 40. The connection line 40 between the first and second blade portions 22, 24 may be a spanwise split, with the connection line 40 being chordwise. The first blade portion 22 extends from the blade root 11 to the connection line 40. The second blade portion 24 extends from the blade connection line 40 to the blade tip 12.

[0060] It will be appreciated that the blade 5 may have any number of blade portions 22, 24, with respective connection joints between them.

[0061] As previously referred to above, the first and second blade portions 22, 24 are coupled by a connection joint that includes a connector 41 as shown in Figure 3. The connector may comprise a first connector portion 41a and a second connector portion 41 b, such as shown in Figure 4. The connector 41 connects a first blade end surface of the first blade portion 22 to a second blade end surface of the second blade portion 24. The connector 41 is adapted to transfer load between the first blade portion 22 and the second blade portion 24, and in particular between a first spar cap portion 23 of the first blade portion 22 and a second spar cap portion 25 of the second blade portion 24.

[0062] The connector 41 may be a cast metallic component, although it will be appreciated that the connector may be formed of any suitable material, e.g. composite materials, and produced by any suitable manufacturing technique, e.g. machined, co-cured or cobonded. The connector 41 may be made from aluminium. The connector 41 is a single unitary connector component, although it will be appreciated that the connector 41 may be formed of two or more components in some examples.

[0063] In the example shown in Figure 3, the connector 41 includes a first branch 54 for connecting the suction side 15 of the first and second blade portions 22, 24 and a second branch 55 for connecting the pressure side 16 of the first and second blade portions 22, 24. The first and second branches 54, 55 may be connected by a first link 56 located towards the leading edge 13 of the blade 5 and connected by a second link

[0064] 57 located towards the trailing edge 14 of the blade 5. In this way, a ring shape is formed by the first branch 54, second branch 55, first link 56 and second link 57. The first and second branches 54, 55 may be integrally formed with the first and second links 56, 57, although it will be appreciated that the first and second branches 54, 55 may be separate components from each other, and / or the first and second links 56, 57. It will be appreciated that the connector 41 may take other forms, for example the second link 57 may be located away from the trailing edge 14 of the blade 5 so as to form a generally ‘A’ shaped connector.

[0065] The connector 41 may extend across substantially the entire chord of the wind turbine blade 5, although preferably the connector 41 extends across only a portion of the chord of the wind turbine blade. This assists in reducing the weight of the connector 41 , whilst allowing the connector 41 to be positioned adjacent the spar cap portions of the blade portions 22, 24 that may carry the majority of the loads. The connector 41a may include a plurality of apertures. In this manner, the first and second blade portions 22, 24 may be attached together with sets of fasteners (not shown) that extend through the holes.

[0066] The connector 41 may extend across any chordwise portion of the blade 5 adjacent the spar cap portions 23, 25.

[0067] The wind turbine blade 5 further comprises a lightning protection system to reduce the likelihood of damage to the blade 5 in the event of a lightning strike. Each of the first 22 and second 24 blade portions comprises a down conductor portion 60, 70 respectively which may for example be formed integrally with the blade shell or retrofitted to an outer surface of the blade shell. Each down conductor portion 60, 70 may comprise a metal foil layer in intimate electrical contact with the blade shell. It will be appreciated that a metal foil layer may here refer to a layer of expanded metal foil or a layer of metal mesh comprising a plurality of apertures through the foil. Alternatively, at least one of the down conductor portions 60, 70 may comprise an electrical cable.

[0068] There is a gap between the connector 41 and at least one of the down conductor portions 60, 70. The gap may be substantially in the spanwise direction of the blade 5 as shown in Figure 3. Although shown in a partially assembled state in Figure 3, it will be appreciated that once assembled there is a gap between the first windward down conductor portion 60 and the connector 41 , and between the second windward down conductor portion 70 and the connector 41 . The size of the gap between the at least one down conductor portion and the connector 41 may be greater than 0.5 metres, optionally greater than 1 metre, further optionally greater than 2 metres.

[0069] In the event of a lightning strike, electric current may seek to arc across the gap between the down conductor portions and the connector 41 . The lightning protection system therefore further comprises an electrical connection in the form of an electrical cable 80 electrically bonding the connector 41 to the at least one down conductor portion across the gap. Where there is a plurality of gaps between the connector 41 and separate down conductor portions, there may be provided a plurality of electrical cables 80 bonding each of the down conductor portions to the connector 41. Electrical bonding is here defined as electrically coupling components such that the electrical connection point on each component has a substantially equal potential, thus minimising the risk of electric current arcing therebetween. In this way, a controlled pathway is provided for lightning current to travel through the lightning protection system and down to ground.

[0070] Figures 3, 4 and 5 show lightning protection systems comprising four down conductor portions each provided as metal foil layers. Each metal foil layer extends over one of: a. The first spar cap portion on the suction side of the first blade section 22 (this may be referred to as a first leeward foil layer 60) b. The first spar cap portion on the pressure side of the first blade section 22 (this may be referred to as a first windward ward foil layer 65) c. The second spar cap portion on the suction side of the second blade section 24 (this may be referred to as a second leeward foil layer 70) d. The second spar cap portion on the pressure side of the second blade section 24 (this may be referred to as a second windward foil layer 75)

[0071] Each down conductor portion is electrically bonded to the connector 41 by an electrical cable 80. In an alternative embodiment (not shown), a down conductor portion may be electrically bonded to the connector 41 via a different down conductor portion. By way of non-limiting example, the first windward foil layer 65 may be electrically bonded to the first leeward foil layer 60 by an electrical cable 80. The first leeward foil layer 60 may be electrically bonded to the connector 41 by a further electrical cable 80. In this way electric current may flow between the first windward foil layer 65, the first leeward foil layer 60 and the connector 41 , thus reducing the likelihood of electric current arcing across gaps therebetween.

[0072] Each electrical cable 80 may be coupled to a down conductor portion at an electrical connection point. Figure 3 shows an example of a first leeward foil layer 60 comprising a first electrical connection point 62 and a second leeward foil layer 70 comprising a second electrical connection point 72. Down conductor portions on the windward side of the blade 5 may also comprise similar electrical connection points such as a third electrical connection point 67 and a fourth electrical connection point 77 as shown in Figure 5b.

[0073] The electrical connection point 62, 67, 72, 77 may comprise an electrically conductive pin which extends through a metal reinforcing disc formed in a metal foil layer. The electrically conductive pin may be used to electrically connect the metal foil layer to the electrical cable 80 of the lightning protection. The electrically conductive pin may form an intimate electrical connection to the metal foil layer via the metal reinforcing disc. This enables lightning current to be conducted via the metal foil layer to ground through the electrically conductive pin. The electrically conductive pin may have a head. An underside of the head may face a surface of the metal reinforcing disc. This may form an intimate electrical connection between the pin head and the metal reinforcing disc. The head of the pin may be substantially flush with the blade shell outer surface. Providing a pin head that is substantially flush with the blade shell outer surface may reduce or negate the effect of the pin on aerodynamic performance of the wind turbine blade. The metal reinforcing disc may be a cast component, cast through the metal foil layer. The metal reinforcing disc may be cast through the apertures of the mesh or foil. Alternatively, the metal reinforcing disc may be assembled to the metal foil layer as two half discs, one on either side of the metal foil layer, and fixed to the metal foil layer, e.g. by soldering or welding for example.

[0074] Preferably, the electrical cables 80 pass through the internal cavity defined by the blade shell such that the electrical cables 80 are protected from the external environment.

[0075] Figure 4 shows a wind turbine blade 5 comprising a connector 41 including a first connector portion 41 a separated from a second connector portion 41b with a chordwise gap therebetween. The first connector portion 41a is located towards the leading edges located towards the trailing edges 14 of the first 22 and second 24 blade portions. As well as the main spar cap portions 23, 24, each blade portion may have in their respective trailing edge regions a longitudinal load bearing structure such as a rear spar or stringer. The second connector portion 41 b may be adapted to transfer load between the trailing edge region of the first blade portion 22 and the trailing edge of the second blade portion 24. The rear spar or stringer portions may also comprise carbon reinforced fibre material and hence may need to be protected by the metal foil layers 60, 70. The metal foil layers 60, 70 may extend chordwise from forward of the (main) spar cap portions 23, 24 to rearward of the rear spar / stringer portions of the respective blade portion.

[0076] Providing the connector 41 as two separate connector portions 41a, 41b each adapted to transfer load between different spar cap portions may reduce the weight of the connection joint. The chordwise gap between the first 41a and second 41 b connector portions may provide an opportunity for electric current to arc between the two connector portions. Therefore, the lightning protection system may further comprise an electrical cable 80 electrically bonding the first connector portion 41a to the second connector portion 41b across the chordwise gap. The electrical cable 80 electrically boning the first connector portion 41a to the second connector portion 41b may be referred to as a connector cable 82 as shown in Figures 4 and 5.

[0077] It may be preferable to reduce the distance or size of the gap between each down conductor portion (in particular the electrical connection point 62, 67, 72, 77) and the connector 41. This may for example allow shorter electrical cables 80 to be used to electrically bond the down conductor portion to the connector 41 . The metal foil layers may therefore comprise a projection extending towards the connector 41 with the electrical connection point 62, 72 provided on the projection. Where the connector 41 comprises a first connector portion 41a spaced apart from a second connector portion 41b as described with reference to Figure 4, the projection may extend towards, optionally into, the gap between the first 41a and second 41b connector portions. The chordwise gap between the two connector portions 41a, 41b may provide a convenient space into which the projections may extend, reducing the overall dimensions of the connection joint and lightning protection system. It will be appreciated that where the lightning protection system comprises a plurality of foil layers, any number of the foil layers may comprise such a projection. Figure 5a shows a blade 5 comprising an example of a first leeward foil layer 60 and a second leeward foil layer 70. Each foil layer 60, 70 comprises a projection 64, 74 respectively which extends towards the chordwise gap between a first connector portion 41a and a second connector portion 41b. It is preferable to provide each projection 64, 74 as a patch of metal foil such that the position of the projection 64, 74 and hence the electrical connection point 62, 72 on the blade portion may be more closely controlled during manufacture. Each projection patch 64, 74 partially overlays and is in intimate electrical contact with a main portion of each respective foil layer 60, 70. Providing the electrical connection point 62, 67, 72, 77 on the patch may allow the position of the patch and hence the electrical connection point to be more accurately controlled, with tolerance being taken up by the amount of overlap between the patch

[0078] 64, 74 and the respective metal foil layer 60. Providing the patch may also improve handling and reduce the cost of the metal foil layer as the metal foil layer can have a more regular shape. Assembling the projection patches 64, 74 and foil layers 60, 70 as such may increase the likelihood of a strong electrical connection between the components thus providing a controlled path through which electric current may flow.

[0079] Although only shown applied to metal foil layers 60, 70 on the leeward surface of the blade 5, it will be appreciated that each foil layer on the windward surface of the blade

[0080] 65, 75 may also comprise such projections, optionally where each projection is formed as a patch of metal foil overlaying and in intimate electrical contact with the windward foil layers.

[0081] Figure 5a shows a first set of inserts 90 comprising a plurality of first inserts 92 embedded in the first spar cap portion 23 and a second set of inserts 100 comprising a plurality of second inserts 102 embedded in the second spar cap portion 25. For clarity only some of the first inserts 92 and second inserts 102 have been labelled with reference numerals. As shown most clearly in Figure 5B, the first blade portion 22 may comprise two first spar cap portions 23, one coupled to each of the suction and pressure surfaces. Similarly, the second blade portion 24 may comprise two second spar cap portions, one coupled to each of the suction and pressure surfaces. The structure of each spar cap portion 23, 25 may be substantially identical.

[0082] Each of the first spar cap portions 23 and the second spar cap portions 24 may comprise a slab 94, 104 respectively arranged adjacent the connection. The slab 94, 104 may be coupled to, optionally embedded in, the blade shell. The slab 94, 104 may comprise glass fibres, and the inserts 92, 102 may be embedded in the slab 94, 104 respectively. Each spar cap portion may further comprise carbon material 96, 106 coupled to the slab 94, 104 and extending away from the connection joint in the spanwise direction. The carbon material 96, 106 may be pultruded carbon or any other carbon reinforced fibre layer, for example a unidirectional or multi-axial fibre layer. The slab preferably has a tapering width which increases from the carbon material 96, 106 towards the connection joint to increase the width of connected joint and reduce stress concentration. The slab 94, 104 may have a taper of reducing height in the blade thickness direction away from the connection joint. The slab 94, 104 may be scarfed or otherwise joined into the carbon material 96, 106 of the spar cap portions 23, 24.

[0083] Each first insert 92 and second insert 102 may include electrically conductive material and may for example include any combination of metallic material, carbon reinforced material and glass fibres. The connector 41 is configured to fasten to the first set of inserts 90 and the second set of inserts 100. Each insert 92, 102 provides an aperture into which a fastener (not shown) may be inserted and coupled, for example using a male-female threaded engagement. Each fastener may be passed through a corresponding aperture in the connector 41 such that the connector 41 is fastened to the first blade portion 22 and the second blade portion 24 via the fasteners. As such each insert 92, 102 is in intimate electrical contact with and hence electrically bonded to the connector 41 .

[0084] As shown most clearly in Figure 5a, there are no inserts 92, 102 directly beneath the metal foil layers 60, 70, beneath here defined as being in the thickness direction from the outside towards the inside of the blade 5. That is to say that there is a gap in each of the first set of inserts 90 and the second set of inserts 100 in the chordwise direction. This chordwise gap may provide a convenient space for the electrical cables 80 extending from the connector 41 to be coupled to the foil layers 60, 70 as shown most clearly in Figure 5b. It is noted that Figure 5b is a cross section view along the line A:A in Figure 5a. Several of the first inserts 92 and the second inserts 102 are shown as hidden details. The connector cable 82 electrically bonding the first connector 41a and the second connector 41 b is not shown for clarity.

[0085] Where the first 23 and / or second 25 spar cap portions include carbon material 96, 106 as described previously, the carbon material is terminated a distance away from the connection joint as shown in Figure 5b. The distance between the connection joint and the end of the carbon material 96, 106 may preferably be occupied by the length of a slab 94, 104 coupling the carbon material to the connector 41.

[0086] Each down conductor portion as a metal foil layer 60, 70 may extend over and beyond the carbon 96, 106 such that the carbon 96, 106 is terminated further away from the connection joint than the metal foil layer 60, 70. Figure 6 shows an example of such an arrangement comprising a plurality of carbon pultrusions 96A, 96B, 96C with the outline of a first foil layer 60 extending over the ends of each carbon pultrusion. For clarity, the first foil layer 60 is provided below the lowermost carbon pultrusion 96C.

[0087] Each carbon pultrusion 96A, 96B, 96C is equipotentially bonded to the first foil layer 60 by an equipotential bonding element 98A, 98B, 98C or “short cut”. It will be appreciated that more generally an equipotential bonding element 98 may be used to equipotentially bond carbon material 96 to a down conductor portion. In this way the equipotential bonding element 58 defines a path between the down conductor first foil layer 60 and the first spar cap portion 23. By electrically bonding the first foil layer 60 to the first spar cap portion 23, the spar cap may be held at the same potential as the first foil layer 60 such that in the event of a lightning strike undesirable discharges or arcs from the lightning conductor to the spar cap can be avoided.

[0088] The equipotential bonding element 98 may comprise a strip or ribbon including electrically conductive material. For example, the equipotential bonding element 98 may comprise strands of electrically conductive material, such as metal wire, woven into a fabric material, such as a glass fibre fabric. In an example the conductive material of the equipotential bonding element 98 may be copper coated with tin, for example. The material may be chosen to avoid any galvanic reaction with either the metal foil layer 60 or the conductive material of the first spar cap portion 23.

[0089] Although described above in relation to a first leeward foil layer 60 equipotentially bonded to carbon material 96, it will be appreciated that the above described equipotential bonding elements 98 may be used to electrically bond any of the foil layers described herein to carbon material.

[0090] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims

Claims

CLAIMS1 . A wind turbine blade comprising: a first blade portion having a shell that defines a suction side, a pressure side, a leading edge, a trailing edge, and a first spar cap portion, the first blade portion further including a first blade portion end surface at one end of the first blade portion; a second blade portion having a shell that defines a suction side, a pressure side, a leading edge, a trailing edge, and a second spar cap portion, the second blade portion further including a second blade portion end surface at one end of the second blade portion, wherein the first blade portion and the second blade portion are configured to be coupled together at the first and second blade portion end surfaces; a connection joint for coupling the first and second blade portions together, wherein the connection joint includes a connector for connecting to the first blade portion end surface and to the second blade portion end surface, the connector including electrically conductive material; and a lightning protection system including a down conductor portion in each of the first and second blade portions, wherein there is a gap between the connector and at least one of the down conductor portions, the lightning protection system further comprising an electrical connection electrically bonding the connector to the at least one down conductor portion across the gap.

2. The wind turbine blade of claim 1 , wherein the electrical connection comprises an electrical cable.

3. The wind turbine blade of claim 1 or claim 2, wherein the down conductor portion of the first and / or second blade portions includes a metal foil layer.

4. The wind turbine blade of claim 3, wherein the lightning protection system includes at least four of the down conductor portions each provided as metal foil layers respectively extending over: the first spar cap portion on the suction side of the first blade portion; the first spar cap portion on the pressure side of the first blade portion; the second spar cap portion on the suction side of the second blade portion; and the second spar cap portion on the pressure side of the second blade portion.

5. The wind turbine blade of any preceding claim, wherein the lightning protection system further includes a plurality of electrical cables electrically bonding each of the down conductor portions to the connector.

6. The wind turbine blade of any preceding claim, wherein the connector is adapted to transfer load between the first spar cap portion of the first blade portion and the second spar cap portion of the second blade portion.

7. The wind turbine blade according to any preceding claim, wherein the connector includes a first connector portion located towards the leading edges of the first and second blade portions and a second connector portion located towards the trailing edges of the first and second blade portions, with a chordwise gap between the first and second connector portions.

8. The wind turbine blade of claim 7, wherein the lightning protection system further comprises an electrical cable electrically bonding the first connector portion to the second connector portion across the chordwise gap.

9. The wind turbine blade according to claim 7 or claim 8, when dependent on claim 3, wherein the metal foil layer has a projection extending towards the chordwise gap between the first and second connector portions, and wherein the electrical cable is coupled to the metal foil layer at the projection.

10. The wind turbine blade according to claim 9, wherein the projection is formed as a patch of metal foil partially overlaying and in intimate electrical contact with a main portion of the metal foil layer.

11. The wind turbine blade of any preceding claim, wherein the first blade portion comprises a first set of inserts embedded in the first spar cap portion, and the second blade portion comprises a second set of inserts embedded in the second spar cap portion, and the connector is for fastening to the first set of inserts and to the second set of inserts, wherein the inserts each include electrically conductive material.

12. The wind turbine blade of claim 11 , wherein the wind turbine blade has a thickness direction and the inserts are not directly beneath the metal foil layer in the thickness direction from the outside towards the inside of the blade.

13. The wind turbine blade of any preceding claim, wherein the first and / or second spar cap portions include carbon fibre reinforced material, which is terminated at a distance away from the connection joint, and the lightning protection system includes a down conductor portion provided as a metal foil layer extending over the carbon fibre reinforced material, preferably wherein the carbon fibre reinforced material is pultruded carbon.

14. The wind turbine blade of claim 13, wherein an equipotential bonding element electrically bonds the metal foil layer to the carbon fibre reinforced material.

15. The wind turbine blade of claim 13 or claim 14, wherein the carbon fibre reinforced material is terminated further away from the connection joint than the metal foil layer.

16. The wind turbine blade of any preceding claim, wherein the first blade portion is an inboard blade part comprising the root end, and the second blade portion is an outboard blade part comprising the tip end.

17. The wind turbine blade of claim 16, wherein the first spar cap portion of the inboard blade part does not comprise any carbon fibre reinforced material and wherein the down conductor portion of the inboard blade part does not comprise a metal foil layer.

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